1. Field of the Invention
The present invention relates to a servo amplifier circuit. In particular, it relates to a servo amplifier circuit having an amplifier portion and a inverting portion which inverts a polarity of an output of the amplifier.
The present invention is applicable to a servo system for a magnetic head in a magnetic disk device.
2. Description of the Related Art
In a magnetic disk device, the magnetic head is moved in two seek modes, a forward seek mode and a reverse seek mode, each corresponding to the direction of the motion of the magnetic head, i.e., corresponding to which side of the target track position the magnetic head exists when the target track position is commanded.
In a servo system for a magnetic head in a magnetic disk device, a servo amplifier circuit having an amplifier portion and an inverting portion which inverts the polarity of the output of the amplifier, is used since a specific non-linear amplifier gain characteristics are used because a fast seeking speed is required when the magnetic head is far from the target track position and a slow seeking speed is required when the magnetic head is near the target track position, and the characteristics are the same in both the forward seek mode and the reverse seek mode without the polarity. The inverting portion operates when the magnetic disk device is in a reverse seek mode.
FIG. 1 shows a construction of the conventional servo amplifier circuit used for a velocity control system for a magnetic head in a magnetic disk device. In FIG. 1, reference numeral 14 denotes a CPU, 25 denotes a target velocity setting circuit, 26 denotes a digital to analog converter, 27 denotes an operational amplifier, 28 denotes a gain characteristics setting circuit, 29 denotes a manual offset adjusting circuit, 30 denotes a forward/reverse conversion circuit, and 50 denotes a velocity control circuit.
The CPU 14 calculates and commands a target value of the velocity of the magnetic head as a digital form of a voltage value, based on the difference between the target track position and the actual track position at that time. The digital to analog converter 26 converts the digital target velocity voltage to an analog form. The analog target velocity voltage is input to an amplifier portion consisting of an operational amplifier, a gain characteristics setting circuit 28, a manual offset adjusting circuit 29, and resistors.
The gain characteristics setting circuit 28 is a resistor having a value which varies with a voltage applied between both terminals of the gain characteristics setting circuit 28, and the varying characteristics are such that the gain of the amplifier portion is large when the magnetic head is far from the target track position and the gain is small when the magnetic head is near the target track position. These characteristics are required because a fast seeking speed is required when the magnetic head is far from the target track position and because a slow seeking speed is required when the magnetic head is near from the target track position.
The manual offset adjusting circuit 29 is provided for manually adjusting an offset of the amplifier portion.
The forward/reverse conversion circuit 30 realizes the above inverting portion, and operates in the reverse seek mode under the control of the CPU 14. The output of the above amplifier portion passes through the forward/reverse conversion circuit 30 without polarity invertion in the forward seek mode, and the output of the above amplifier portion is inverted at the forward/reverse conversion circuit 30 in a reverse seek mode. The output of the forward/reverse conversion circuit 30 is input to the velocity control circuit 50 as a target voltage.
The velocity control circuit 50 detects the actual velocity of the magnetic head as a voltage, obtains a deviation of the actual velocity voltage from the target voltage, and then controls an current for driving a motor.
It is well-known that an analog circuit has an offset in its output, for example, an amplifier circuit comprising an operational amplifier has an offset in its output voltage, i.e., it has a non-zero voltage output (offset) even when its input voltage is zero.
Due to an offset generated in the amplifier portion comprising the operational amplifier 27, the gain characteristics setting portion 28, the manual offset adjusting portion 29, and the resistors, and another offset generated in the inverting portion 30, the output voltage of a servo amplifier circuit having a construction shown in FIG. 1, has characteristics as shown in FIG. 2 or FIG. 3.
In FIGS. 2 and 3, a curve which is denoted by "FWD seek" shows an output voltage of the servo amplifier circuit of FIG. 1 in the forward seek mode, and a curve which is denoted by "REV seek" shows an output voltage of the servo amplifier circuit of FIG. 1 in the reverse seek mode, as a function of the position of the magnetic head, and the position "O" denotes a target track position of the magnetic head.
In FIGS. 2 and 3, the overall position of the two curves are shifted with reference to the zero volt line due to the offset generated in the following stage of the amplifier portion in the construction of FIG. 1.
When the offset generated in the amplifier portion has the same polarity as the polarity of the remaining component (other than the offset) of a non-zero output of the amplifier portion, i.e., positive, the two curves corresponding to the forward seek and the reverse seek are apart from each other as shown in FIG. 2, and when the offset generated in the amplifier portion has the opposite polarity to the polarity of the remaining component (other than the offset) of a non-zero output of the amplifier portion, i.e., positive, the two curves corresponding to the forward seek and the reverse seek intersect as shown in FIG. 3.
Further, in FIGS. 2 and 3, the above curves of the FWD seek and the REV seek continue to the target track position, and each voltage in the forward seek mode and the reverse seek mode has a non-zero value, respectively denoted by V.sub.F and V.sub.R.
Generally, in a sevo system of a magnetic head which is used in a magnetic disk device, the control is changed from the velocity servo control as mentioned above to a fine position control when the magnetic head approaches the target track position to a predetermined degree. Therefore, the above non-zero values V.sub.F and V.sub.R in the vicinity of the target track position in the velocity servo affect a positioning accuracy of the magnetic head. In particular, the difference between the values V.sub.F and V.sub.R results in different positioning accuracy in the forward seek mode and the reverse seek mode.
Therefore, it is necessary to reduce the above difference between the output voltage values V.sub.F and V.sub.R in the vicinity of the target track position, i.e., at the target track position in the velocity servo stage.
For the above purpose, the manual offset adjust circuit 29 is provided. The operator can manually adjust the output level of the above-mentioned amplifier portion at the manual offset adjust circuit 29, and the output voltages of the servo amplifier circuit in the forward seek mode and the reverse seek mode are changed as shown in FIG. 4, and thus the characteristics of the output voltage of the servo amplifier circuit as shown in FIG. 5 can be obtained.
However, the manual adjusting of offset is troublesome, and the manual adjusting of offset cannot follow the changes of the characteristics of the analog circuit due to a deterioration due to age or temperature variation.
Otherwise, it may be possible to reduce the offset by using high-precision components for constructing the circuit. However, this causes an increase in cost.